UPDATE: Charging works, LEDs do not.
I started doing a lot with low power devices (Arduino, BLE 4.0) and coin cells are a convenient power source. Lately I found out that there are even rechargeable coin cells. That's why I needed a charger. There are only a few models out there which were either expensive or I didn't trust the charge current (too high). So I made my own.
It is based on the MCP73831. Order three PCBs for 13.60$ at OSHPARK: https://oshpark.com/projects/roORU8h6
There are two charge modes:
- fast (45mA)
- slow (25mA)
Max charge current for a coin cell is usually between 80% and two third of it's capacity. So you could charge a 100mA coin cell with 80mA. I wanted to increase the lifetime of my coin cells and reduced the charge current. If you want to have different currents here is the equation to calculate R_SLOW/R_FAST:
mA = 1000 / R in kΩ